Memory system capable of reducing the reading time

ABSTRACT

A bias circuit, a memory system, and a method of boosting a voltage level of a first bit line are provided. The bias circuit includes a first current generator, a second current generator, and a bit line bias generator. The first current generator is configured to generate a first replica charging current according to a charging current flowing through a voltage bias transistor. The second current generator is configured to generate a first replica cell current according to a cell current flowing through a common source transistor. The bit line bias generator is coupled to a first page buffer, the first current generator, and the second current generator, and configured to generate a bit line bias voltage, supplied to the first page buffer, according to a comparison of the first replica charging current and the first replica cell current.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/187,679, filed on Feb. 26, 2021, which is a continuation of U.S.application Ser. No. 16/436,926, filed on Jun. 11, 2019, which is acontinuation of International Application No. PCT/CN2019/085219, filedon Apr. 30, 2019, all of which are incorporated herein by reference intheir entireties.

BACKGROUND

The present disclosure is related to a memory system, and moreparticularly to a memory system capable of reducing the reading time.

In a memory system, the data stored in the memory cell is usually readby sensing the data voltage on the bit line caused by the memory cell.For example, in a NAND memory read sequence, to read the data stored ina memory cell, the bit line coupled to the memory cell may bepre-charged to a predetermined level first. After the voltage of the bitline has settled, the word line coupled to the memory cell may be raisedto cause the memory cell to generate current according to the datastored in the memory cell. If the memory cell has not been programmed,the memory cell may generate a significant current that pulls down thevoltage of the bit line. Otherwise, if the memory cell has beenprogrammed, the memory cell will not generate any currents or will onlygenerate insignificant current so the voltage of the bit line willremain at the similar level. Therefore, by sensing the voltage of thebit line, the data stored in the memory cell can be read.

However, since the bit line is resistive and capacitive due toinevitable parasitic resistors and capacitors, the settling time of thebit line will contribute to a significant part of the total readingtime. Furthermore, since the resistive and capacitive characteristicsare unpredictable and varied with process, the settling time required bydifferent memory cells are also different. Therefore, the worst casesettling time is always applied to ensure the sensing accuracy. Inaddition, in prior art, the bit line is pre-charged with a master-slavetransistor controlled by a predetermined voltage. In this case, thecharging ability may decrease as the voltage of the bit line approachingto the desired level, which also increases the reading time.

SUMMARY

One embodiment of the present disclosure discloses a memory system. Thememory system includes a plurality of memory cells, a voltage biastransistor, a page buffer, a common source transistor, and a biascircuit.

The first memory cells are coupled to a bit line. The voltage biastransistor has a first terminal for receiving a first system voltage, asecond terminal, and a control terminal for receiving a first biasvoltage.

The page buffer is coupled to the bit line and the second terminal ofthe voltage bias transistor. The page buffer charges the first bit lineto the first system voltage according to a bit line bias voltage duringa pre-charge operation, and forms a sensing path from the first bit lineto a sensing amplifier during a sense operation.

The common source transistor has a first terminal coupled to the firstbit line, a second terminal for receiving a second system voltagesmaller than the first system voltage, and a control terminal forreceiving a control signal.

The bias circuit includes a charging current reproduce unit, a cellcurrent reproduce unit, a current comparator, and a bit line biasgenerator. The charging current reproduce unit is coupled to the voltagebias transistor. The charging current reproduce unit generates acharging reference voltage according to a charging current flowingthrough the voltage bias transistor. The cell current reproduce unit iscoupled to the common source transistor. The cell current reproduce unitgenerates a cell reference voltage according to a cell current flowingthrough the common source transistor.

The current comparator is coupled to the charging current reproduce unitand the cell current reproduce unit. The current comparator includes afirst current generator, and a second current generator. The firstcurrent generator generates a replica charging current according to thecharging reference voltage, and the second current generator generates areplica cell current according to the cell reference voltage.

The bit line bias generator is coupled to the current comparator and thefirst page buffer. The bit line bias generator generates the bit linebias voltage according to a difference between the first replicacharging current and the first replica cell current.

Another embodiment of the present disclosure discloses a bias circuit.The bias circuit includes a charging current reproduce unit, a cellcurrent reproduce unit, a current comparator, and a bit line biasgenerator.

The charging current reproduce unit is coupled to a voltage biastransistor, and generates a charging reference voltage according to acharging current flowing through the voltage bias transistor. The cellcurrent reproduce unit is coupled to a common source transistor, andgenerates a cell reference voltage according to a cell current flowingthrough the common source transistor.

The current comparator is coupled to the charging current reproduce unitand the cell current reproduce unit. The current comparator includes afirst current generator, and a second current generator. The firstcurrent generator generates a replica charging current according to thecharging reference voltage, and the second current generator generates areplica cell current according to the cell reference voltage.

The bit line bias generator is coupled to the current comparator and apage buffer, and generates a bit line bias voltage to control the pagebuffer for charging a bit line according to a difference between thereplica charging current and the replica cell current.

The plurality of first memory cells are coupled to the bit line, thevoltage bias transistor has a first terminal for receiving a firstsystem voltage, a second terminal, and a control terminal for receivinga first bias voltage. The page buffer is coupled to the bit line and thesecond terminal of the voltage bias transistor, and charges the bit lineto the first system voltage according to the bit line bias voltageduring a pre-charge operation. The common source transistor has a firstterminal coupled to the bit line, a second terminal for receiving asecond system voltage smaller than the first system voltage, and acontrol terminal for receiving a control signal.

These and other objectives of the present disclosure will no doubtbecome obvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a memory system according to one embodiment of the presentdisclosure.

FIG. 2 shows a bias circuit according to one embodiment of the presentdisclosure.

DETAILED DESCRIPTION

FIG. 1 shows a memory system 100 according to one embodiment of thepresent disclosure. The memory system 100 includes a plurality of memorycells MC(1,1) to MC(M,N), a voltage bias transistor 110, page buffers1201 to 120N, a common source transistor 130, and a bias circuit 140,where M and N are positive integers.

In FIG. 1, the memory cells MC(1,1) to MC(M,N) are disposed as an array.For example, the memory cells MC(1,1) to MC(M,1) can be coupled to thebit line BL1, and the memory cells MC(N) to MC(M,N) can be coupled tothe bit line BLN. Also, the memory cells MC(1,1) to MC(N) can be coupledto the word line WL1, and the memory cells MC(M,1) to MC(M,N) can becoupled to the word line WLM.

The voltage bias transistor 110 has a first terminal for receiving afirst system voltage VS1, a second terminal, and a control terminal forreceiving a first bias voltage VB1. The first bias voltage VB1 can turnon the voltage bias transistor 110 to charge the bit lines BL1 to BLNthrough the page buffers 1201 to 120N.

The page buffers 1201 to 120N can have the same structure. For example,the page buffer 1201 can be coupled to the bit line BL1 and the secondterminal of the voltage bias transistor 110. The page buffer 1201 cancharge the bit line BL1 to the first system voltage VS1 according to abit line bias voltage VBLB during the pre-charge operation, and can forma sensing path from the bit line BL1 to a sensing amplifier during thesense operation.

In FIG. 1, the page buffer 1201 includes transistors M1 to M5. Thetransistor M1 has a first terminal coupled to the second terminal of thevoltage bias transistor 110, a second terminal, and a control terminalfor receiving a pre-charge control signal SIG_(C1). The transistor M2has a first terminal coupled to the second terminal of the transistorM1, a second terminal, and a control terminal for receiving a clampingcontrol signal SIG_(C2). The transistor M3 has a first terminal coupledto the second terminal of the second transistor M2, a second terminalcoupled to the bit line BL1, and a control terminal for receiving thebit line bias voltage VBLB. The transistor M4 has a first terminalcoupled to the second terminal of the transistor M2, a second terminalcoupled to the sensing amplifier for sensing, and a control terminal forreceiving a sensing control signal SIG_(C3). The transistor M5 has afirst terminal coupled to the second terminal of the transistor M1, asecond terminal coupled to the second terminal of the transistor M4, anda control terminal for receiving a pre-charge select signal SIGc4.

During the pre-charge operation, the transistors M1 and M2 will beturned on, and the transistor M3 will also be turned on to charge thebit line BL1. In some embodiments, the memory system 100 can furtherinclude high voltage passing transistors 1501 to 150N, and the pagebuffers 1201 to 120N can be coupled to the bit lines BL1 to BLN throughthe high voltage passing transistors 1501 to 150N respectively. In thiscase, the high voltage passing transistor 1501 will also be turned on bythe pass signal SIGHV during the pre-charge operation of the bit lineBL1.

Also, during the sense operation, the transistors M1, M2, and M3 may beturned off, and the transistor M4 can be turned on so that the voltageof the bit line BL can be sensed by the sense amplifier. The transistorM5 can be used to select the bit line to be pre-charged according to therequirement.

The common source transistor 130 has a first terminal coupled to the bitlines BL1 to BLN, a second terminal for receiving a second systemvoltage VS2 smaller than the first system voltage VS1, and a controlterminal for receiving a control signal SIG_(ACS).

During the pre-charge operation of the bit line BL1, the voltage biastransistor 110 and the common source transistor 130 can be turned on,and the transistors M1, M2, and M3 of the page buffer 1201 can also beturned on. Therefore, the bit line BL1 can be pre-charged. However, inprior art, as the voltage of the bit line BL1 increases , thegate-to-source voltage applied on the transistor M3 will decrease,thereby weakening the charging ability and increasing the required timefor pre-charging. In the memory system 100, to address this issue, thebias circuit 140 can be used to generate and adjust the bit line biasvoltage VBLB for controlling the transistor M3 according to thecondition of the pre-charging operation.

FIG. 2 further shows the bias circuit 140 according to one embodiment ofthe present disclosure. The bias circuit 140 includes a charging currentreproduce unit 142, a cell current reproduce unit 144, a currentcomparator 146, and a bit line bias generator 148.

The charging current reproduce unit 142 is coupled to the voltage biastransistor 110, and can generate a charging reference voltage Vref1according to a charging current Ichg flowing through the voltage biastransistor 110.

The cell current reproduce unit 144 is coupled to the common sourcetransistor 130, and can generate a cell reference voltage Vref2according to a cell current Icell flowing through the common sourcetransistor 130.

The current comparator 146 is coupled to the charging current reproduceunit 142 and the cell current reproduce unit 144. The current comparator146 includes a first current generator 146A, and a second currentgenerator 146B. The first current generator 146A can generate a replicacharging current litho according to the charging reference voltageVref1, and the second current generator 146B can generate a replica cellcurrent I_(rcell1) according to the cell reference voltage Vref2.

The bit line bias generator 148 is coupled to the current comparator 146and the page buffers 1201 to 120N. The bit line bias generator 148 cangenerate the bit line bias voltage VBLB according to a differencebetween the replica charging current litho and the replica cell currentI_(rcell1).

In some embodiments, part of the charging current Ichg flowing throughthe voltage bias transistor 110 may flow to the parasitic capacitors onthe bit lines BL1 to BLN in the beginning of the pre-charge operationwhile the rest of the charging current Ichg will flow through the commonsource transistor 130. Later, when the parasitic capacitors are charged,the charging current Ichg will all flow through the common sourcetransistor 130.

That is, in the beginning of the pre-charge operation, the chargingcurrent Ichg would be greater than the cell current I_(rcell1), and,thus, the replica charging current I_(rchg1) should be greater than thereplica cell current I_(rcell1). In this case, the difference betweenthe replica charging current I_(rchg1) and the replica cell currentI_(rcell1) will cause the bit line bias generator 148 to raise the bitline bias voltage VBLB so the transistor M3 can be fully turned on,thereby increasing the charging ability.

Later, when the parasitic capacitors are charged completely, the replicacharging current T_(rchg1) will be substantially equal to the replicacell current I_(rcell1). In this case, it may imply that the bit lineBL1 has been charged so the bit line bias generator 148 will keep thebit line bias voltage VBLB, and the sense operation can be performedcorrespondingly.

In some embodiments, the current comparator 146 can further include athird current generator 146C, a fourth current generator 146D, and aninverter 146E for generating a sensing indication signal SIG_(IDCT). Thethird current generator 146C can generate a replica charging currentI_(rchg2) according to the charging reference voltage Vref1, and thefourth current generator 146D can generate a replica cell currentI_(rcell2) according to the cell reference voltage Vref2. The inverter146E has an input terminal coupled to the third current generator 146Cand the fourth current generator 146D, and an output terminal foroutputting the sensing indication signal SIG_(IDCT) according to thedifference between the replica charging current I_(rchg2) and thereplica cell current I_(rcell2). In this case, the sensing indicationsignal SIG_(IDCT) will be flipped when the difference between thereplica charging current I_(rchg2) and the replica cell currentI_(rcell2) becomes zero, and the sense operation can be triggered by theflipped sensing indication signal SIG_(IDCT) accordingly.

Since the bit line bias generator 148 can adjust the bit line biasvoltage VBLB according to the charging status of the bit lines BL1 toBLN instantly, the charging ability can be maintained to be strongduring the pre-charge operation. Also, since the charging status of thebit lines BL1 to BLN can be detected by the difference between thereplica charging current litho and the replica cell current I_(rcell1),the pre-charge operation can be terminated, and the sense operation canbe triggered once the bit lines BL1 to BLN are pre-charged. That is, thepre-charge time can be optimized, and the pre-charge operation can becontrolled without being affected by the process variation.

In FIG. 2, the charging current reproduce unit 142 includes transistorsM6 and M7, and an operational amplifier OP1. The transistor M6 has afirst terminal for receiving the first system voltage VS1, a secondterminal, and a control terminal coupled to the control terminal of thevoltage bias transistor 110. The operational amplifier OP1 has apositive input terminal coupled to the second terminal of the transistorM6, a negative input terminal coupled to the second terminal of thevoltage bias transistor 110, and an output terminal for outputting thecharging reference voltage Vref1. The transistor M7 has a first terminalcoupled to the second terminal of the transistor M6, a second terminalfor receiving the second system voltage VS2, and a control terminalcoupled to the output terminal of the operational amplifier OP1.

In this case, the operational amplifier OP1 can ensure the transistor M6to be biased under the same condition as the voltage bias transistor110. Therefore, the charging current reproduce unit 142 is able togenerate a reproduce current according to the charging current Ichgflowing through the voltage bias transistor 110.

Similarly, the cell current reproduce unit 144 includes transistors M8and M9, and an operational amplifier OP2. The transistor M8 has a firstterminal for receiving the first system voltage VS1, a second terminal,and a control terminal. The operational amplifier OP2 has a positiveinput terminal coupled to the second terminal of the transistor M8, anegative input terminal coupled to the bit lines BL1 to BLN, and anoutput terminal coupled to the control terminal of the transistor M8 foroutputting the cell reference voltage Vref2. The transistor M9 has afirst terminal coupled to the second terminal of the transistor M8, asecond terminal for receiving the second system voltage VS2, and acontrol terminal coupled to the control terminal of the common sourcetransistor 130.

In this case, the operational amplifier OP2 can ensure the transistor M9to be biased under the same condition as the common source transistor130. Therefore, the cell current reproduce unit 144 is able to generatea reproduce current according to the cell current Icell flowing throughthe common source transistor 130.

In FIG. 2, the first current generator 146A includes a transistor M10having a first terminal, a second terminal for receiving the secondsystem voltage VS2, and a control terminal for receiving the chargingreference voltage Vref1. Also, the second current generator 146Bincludes a transistor M11 having a first terminal for receiving thefirst system voltage VS1, a second terminal coupled to the firstterminal of the transistor M10, and a control terminal for receiving thecell reference voltage Vref2.

In addition, in FIG. 2, the transistors M7 and M10 are N-typetransistors while the transistors M8 and M11 are P-type transistors. Inthis case, the transistor M10 will be biased under the same condition asthe transistor M7 with the charging reference voltage Vref1, so thetransistor M10 can generate the replica charging current I_(rchg1) bymirroring the current flowing through the transistor M7. Similarly, thetransistor M11 will be biased under the same condition as the transistorM8 with the cell reference voltage Vref2, so the transistor M11 cangenerate the replica cell current I_(rcell1) by mirroring the currentflowing through the transistor M8.

In FIG. 2, the bit line bias generator 148 includes an operationalamplifier OP3, a transistor M12, and a resistor R1. The operationamplifier OP3 has a positive input terminal for receiving a second biasvoltage VB2, a negative input terminal coupled to the first terminal ofthe transistor M10, and an output terminal for outputting the bit linebias voltage VBLB. The transistor M12 has a first terminal coupled tothe output terminal of the operation amplifier OP3, a second terminalcoupled to the negative input terminal of the operation amplifier OP3,and a control terminal coupled to the first terminal of the transistorM12. The resistor R1 has a first terminal coupled to the second terminalof the transistor M12, and a second terminal for receiving the secondsystem voltage VS2.

In this case, when the replica charging current I_(rchg1) is greaterthan the replica cell current I_(rcell1), a differential currentI_(diff) will be fed to the bit line bias generator 148, thereby pullingdown the voltage of the negative input terminal of the operationalamplifier OP3 and raising the bit line bias voltage VBLB.

In some embodiments, the ratio of the size of the transistors M7 and M10can be selected according to the system requirement to adjust thereplica charging current I_(rchg1). However, the ratio of the size ofthe transistors M8 and M11 should be the same as the ratio of the sizeof the transistors M7 and M10.

Similarly, the ratio of the size of the transistor M6 and the voltagebias transistor 110 can be selected according to the system requirement,and the ratio of the size of the transistor M6 and the voltage biastransistor 110 should be the same as the ratio of the size of thetransistor M9 and the common source transistor 130.

Furthermore, in FIG. 2, the charging current reproduce unit 142 and thecell current reproduce unit 144 can use the operational amplifiers OP1and OP2 to fix the bias conditions firmly; however, in some otherembodiments, the charging current reproduce unit 142 and the cellcurrent reproduce unit 144 may be implemented with other structures,such as the commonly used current mirrors.

Also, in FIG. 1, the bit lines BL1 to BLN can be pre-charged at the sametime, however, in some other embodiments, the bit lines BL1 to BLN mayalso be pre-charged independently with the page buffers 1201 to 120Naccording to the system requirement.

In summary, the memory system and the bias circuit provided by theembodiments of the present disclosure can adjust the bit line biasvoltage according to the charging status of the bit lines instantly, sothe charging ability can be maintained to be strong during thepre-charge operation. Also, since the charging status of the bit linescan be detected by the difference between the replica charging currentand the replica cell current, the pre-charge time can be optimized, andthe pre-charge operation can be controlled without being affected by theprocess variation.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the disclosure. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A bias circuit, coupled to a voltage biastransistor and a common source transistor and comprising: a firstcurrent generator configured to generate a first replica chargingcurrent according to a charging current flowing through the voltage biastransistor; a second current generator configured to generate a firstreplica cell current according to a cell current flowing through thecommon source transistor; and a bit line bias generator coupled to afirst page buffer, the first current generator, and the second currentgenerator, and configured to generate a bit line bias voltage, suppliedto the first page buffer, according to a comparison of the first replicacharging current and the first replica cell current, wherein in responseto the bit line bias voltage, the first page buffer charges a first bitline of a plurality of memory cells.
 2. The bias circuit of claim 1,wherein: the first page buffer comprises a first transistor comprising acontrol terminal controlled by the bit line bias voltage, a firstterminal configured to receive a first system voltage, and a secondterminal coupled to the first bit line; and in response to the firstreplica charging current being greater than the first replica cellcurrent, the bit line bias generator boosts the bit line bias voltage toturn on the first transistor, thereby charging the first bit line to thefirst system voltage.
 3. The bias circuit of claim 1, wherein: inresponse to the first replica charging current being substantially equalto the first replica cell current, the bit line bias generator maintainsthe bit line bias voltage.
 4. The bias circuit of claim 1, wherein: thebit line bias generator comprises a first operational amplifiercomprising a positive input terminal configured to receive a first biasvoltage and a negative input terminal; and in response to the firstreplica charging current being greater than the first replica cellcurrent, a differential current is fed to the negative input terminal ofthe first operational amplifier, and a voltage at the negative inputterminal of the first operational amplifier is pulled down, therebyboosting the bit line bias voltage, wherein the differential currentbeing determined according to the first replica charging current and thefirst replica cell current.
 5. The bias circuit of claim 1, furthercomprising: a charging current reproduce unit coupled to the voltagebias transistor and the first current generator, and configured togenerate a charging reference voltage according to the charging currentflowing through the voltage bias transistor, the first replica chargingcurrent being generated according to the charging reference voltage; anda cell current reproduce unit coupled to the common source transistorand the second current generator, and configured to generate a cellreference voltage according to the cell current flowing through thecommon source transistor, the first replica cell current being generatedaccording to the cell reference voltage.
 6. The bias circuit of claim 5,further comprising: a third current generator coupled to the chargingcurrent reproduce unit and configured to generate a second replicacharging current according to the charging reference voltage; a fourthcurrent generator coupled to the cell current reproduce unit andconfigured to generate a second replica cell current according to thecell reference voltage; and an inverter coupled to the third currentgenerator and the fourth current generator, and configured to output asensing indication signal according to the second replica chargingcurrent and the second replica cell current, the sensing indicationsignal being configured to trigger a sensing operation.
 7. The biascircuit of claim 5, wherein: the voltage bias transistor comprises afirst terminal configured to receive a first system voltage, a secondterminal, and a control terminal; and the charging current reproduceunit comprises: a second transistor comprising a first terminalconfigured to receive the first system voltage, a second terminal, and acontrol terminal coupled to the control terminal of the voltage biastransistor; and a second operational amplifier comprising a positiveterminal coupled to the second terminal of the second transistor, anegative terminal coupled to the second terminal of the voltage biastransistor, thereby ensuring the second transistor to be biased as thevoltage bias transistor to output the charging reference voltage to anoutput terminal of the second operational amplifier.
 8. The bias circuitof claim 7, wherein: the charging current reproduce unit comprises athird transistor comprising a first terminal coupled to the secondterminal of the second transistor, a second terminal configured toreceive a second system voltage, and a control terminal coupled to theoutput terminal of the second operational amplifier; and the firstcurrent generator comprises a fourth transistor comprising a firstterminal coupled to the bit line bias generator, a second terminalconfigured to receive the second system voltage, and a control terminalcoupled to the output terminal of the second operational amplifier, thefourth transistor being configured to mirror a current flowing throughthe third transistor.
 9. A memory system, comprising: a voltage biastransistor coupled to a first page buffer; a common source transistorcoupled to a first bit line of a plurality of first memory cells; and abias circuit coupled to the voltage bias transistor, the common sourcetransistor, and the first page buffer, and configured to: generate afirst replica charging current according to a charging current flowingthrough the voltage bias transistor; generate a first replica cellcurrent according to a cell current flowing through the common sourcetransistor; and generate a bit line bias voltage, supplied to the firstpage buffer, according to a comparison of the first replica chargingcurrent and the first replica cell current, wherein in response to thebit line bias voltage, the first page buffer charges the first bit line.10. The memory system of claim 9, wherein: the voltage bias transistorcomprising a first terminal configured to receive a first system voltageand a second terminal coupled to the first page buffer; the commonsource transistor comprising a first terminal coupled to the first bitline and a second terminal configured to receive a second systemvoltage; and the first system voltage is greater than the second systemvoltage.
 11. The memory system of claim 9, further comprising: the firstpage buffer comprises a first transistor comprising a control terminalcontrolled by the bit line bias voltage, a first terminal configured toreceive a first system voltage, and a second terminal coupled to thefirst bit line, wherein in response to the first replica chargingcurrent being greater than the first replica cell current, the biascircuit boosts the bit line bias voltage to turn on the firsttransistor, thereby charging the first bit line to the first systemvoltage.
 12. The memory system of claim 9, wherein: in response to thefirst replica charging current being substantially equal to the firstreplica cell current, the bias circuit maintains the bit line biasvoltage.
 13. The memory system of claim 9, wherein: in response to thefirst replica charging current being greater than the first replica cellcurrent, the bias circuit generates the bit line bias voltage accordingto a difference between the first replica charging current and the firstreplica cell current.
 14. The memory system of claim 9, wherein the biascircuit comprises an operational amplifier configured to compare thefirst replica charging current and the first replica cell current andoutput the bit line bias voltage to the first page buffer according tothe comparison of the first replica charging current and the firstreplica cell current.
 15. The memory system of claim 9, wherein the biascircuit is further configured to: generate a charging reference voltageaccording to the charging current flowing through the voltage biastransistor, and generate the first replica charging current according tothe charging reference voltage; and generate a cell reference voltageaccording to the cell current flowing through the common sourcetransistor, and generate the first replica cell current according to thecell reference voltage.
 16. The memory system of claim 15, wherein thebias circuit is further configured to: generate a second replicacharging current according to the charging reference voltage; generate asecond replica cell current according to the cell reference voltage; andoutput a sensing indication signal according to the second replicacharging current and the second replica cell current, the sensingindication signal being configured to trigger a sensing operation. 17.The memory system of claim 15, wherein the bias circuit comprises: asecond transistor biased as the voltage bias transistor to generate thecharging reference voltage; and a third transistor biased as the commonsource transistor to generate the cell reference voltage.
 18. A methodof boosting a voltage level of a bit line, comprising: reproducing,through a bias circuit, a charging current flowing through a voltagebias transistor as a first replica charging current, the voltage biastransistor being coupled to a page buffer; reproducing, through the biascircuit, a cell current flowing through a common source transistor as afirst replica cell current, the common source transistor being coupledto the bit line of a plurality of memory cells; generating, through thebias circuit, a bit line bias voltage, supplied to the page buffer,according to a comparison of the first replica charging current and thefirst replica cell current; and in response to the bit line biasvoltage, charging, through the page buffer, the bit line.
 19. The methodof claim 18, wherein: in response to the first replica charging currentbeing greater than the first replica cell current, generating the bitline bias voltage according to a difference between the first replicacharging current and the first replica cell current.
 20. The method ofclaim 18, wherein: generating a charging reference voltage according tothe charging current flowing through the voltage bias transistor, andgenerating the first replica charging current according to the chargingreference voltage; and generating a cell reference voltage according tothe cell current flowing through the common source transistor, andgenerating the first replica cell current according to the cellreference voltage.